1. Field of the Invention
The present invention relates to a synchronous detector and a method therefor. The present invention relates in particular to a symbol timing synchronous detector circuit to be mounted on a demodulator for use in a burst transmission system, such as a wireless local area network (LAN) based upon an orthogonal frequency division multiplexing (OFDM) modulation system. Further, the present invention relates to a method of detecting symbol synchronization used for example in demodulation by an OFDM demodulator.
2. Description of the Background Art
A communication terminal for use in a wireless LAN transmits and receives signals in a packet mode in which signals are conveyed by packets to be transmitted. In a packet mode, a preamble is provided at the top part of the packet. The preamble is used for synchronizing frequency and symbol timing.
A packet mode will be further described. An OFDM packet prescribed in IEEE (Institute of Electrical and Electronics Engineers) 802.11a, the international standards for wireless LAN, includes a preamble, signals and data. The preamble includes a short preamble including short symbols and a long preamble including long symbols. The short preamble includes ten short symbols forming a specific pattern and contributes to establishment of synchronizing frequency, symbol timing and carrier frequency, etc. in the OFDM system. Also, the symbol timing synchronization is established by detecting OFDM symbol timing. Such synchronization is used in demodulation of OFDM burst signals.
The short symbols are arranged followed by the long symbols. The long symbols are used in channel estimation in which a propagation channel is estimated for estimating the phase and amplitude of each sub-carrier distorted on the propagation channel.
There is a method of utilizing a preamble based on such a specific format to establish symbol timing synchronization. The method performs operation of complex correlation using a received signal and a known specific pattern, and compares an obtained correlation value with a predetermined threshold value to detect a peak of the correlation from the comparison. The correlation peak detection corresponds to detection of each of the ten short symbols included in the short preamble. The symbol timing synchronization means to detect a boundary position between a short preamble and a long preamble as synchronization timing so as to obtain a sync detection signal. In order to obtain a sync detection signal, the method further determines, after the elapse of a predetermined period of time from the correlation peak detection, whether or not a correlation peak is detected. When a correlation peak is not detected, then it is determined that the reception of the packet has moved to its long preamble to determine its boundary position, thereby outputting a symbol timing sync signal. The symbol timing synchronization is established in that procedure. A predetermined period of time is in the context determined as corresponding for example to a period of time for a short preamble.
In the determination according to the method stated above, symbol timing synchronization is established on the basis of the failure to detect a correlation peak at an expected timing. It is therefore desirable to correctly detect a correlation peak with a symbol immediately preceding thereto in the repetitive pattern included in a short preamble, i.e. with the tenth short symbol. If the correlation peak should not correctly be detected, symbol timing synchronization cannot be established at a correct timing. As seen from this method in which timing is obtained from the comparison between a correlation value calculated by correlation peak detection and a predetermined threshold value, it is important for detection of a correct correlation peak to set a predetermined threshold value.
In view of the importance stated above, it could be considered that setting a predetermined threshold value to a lower level facilitates detection of a correlation peak. However, it has been known that when a predetermined threshold value is set to such a lower level, higher accuracy cannot be attained on symbol timing synchronization. Such setting tends to render synchronization detection too sensitive to noise and multiple pass on a propagation channel. In particular, the latter, multiple pass propagation, makes the detection difficult due to the distortion of signal waveforms caused by multi-superimposition of delayed incoming waves. As a result, such setting increases the probability in detecting a correlation peak at incorrect timing, which causes to deteriorate the accuracy in symbol timing synchronization.
An OFDM demodulator disclosed in U.S. Pat. No. 6,646,980 B1 to Yamamoto et al detects a short symbol and narrows a detecting window step by step on the basis of the detection to determine timing not exceeding a predetermined threshold value, thereby improving the accuracy in the timing detection.
Further, an OFDM demodulator in U.S. Pat. No. 6,658,063 B1 to Mizoguchi et al, which has, in particular, three threshold circuits provided in a timing decision circuit. Two of the threshold circuits determine whether or not a preamble signal exists before one or two unit periods of time, when the respective input signals in the threshold circuits exceed the threshold value, while the remaining threshold circuit determines whether or not values indicated by the current output signals from a correlator have lowered by a predetermined percent or more compared to the values before one unit period of time. A logic circuit decides, when the result from all those threshold circuits are true, that a time position of the end of the preamble has been detected, thereby providing a highly accurate detection of symbol timing synchronization.
Both documents disclose that an examination on time conditions in synchronization detection could raise accuracy in timing detection. However, it discloses or suggests nothing about determination based on a signal level.